1. Field of the Invention
The present invention relates to electrical measurement equipment and, in particular, to automatic range-changing circuits for use in measuring electrical current.
2. Description of Related Art
A conventional method for determining electrical current, the so-called feedback method, is shown in FIG. 4. A load device, such as a device under test (DUT) 100, draws current Iout from the output of an amplifier 101. The current Iout develops a voltage VI across a resistor 102. The current Iout is proportional to the voltage VI and can be determined based on a measurement of VI across the resistor 102.
The saturation voltage of the amplifier 101 limits the level of current Iout that can accurately be measured using the circuit of FIG. 4. As voltage VI rises due to increased current Iout draw by the DUT 100, the saturation voltage of the amplifier 101 may be approached. Adding additional resistors that can be selectively switched into and out of the feedback circuit of the amplifier 101 will provide ranges of current that can be measured while maintaining the voltage VI within a desirable voltage range below the saturation voltage of the amplifier 101. Such a circuit is shown in FIG. 5. A plurality of switches 104a, 104b, 104c respectively switch resistors R0 106, R1 108, and Rn 110 into and out of the amplifier's 101 feedback circuit. Each resistor 106, 108, 110 is sized to provide a voltage measurement within the desirable range, whatever that may be, for a particular range of current Iout. The switches 104a, 104b, 104c are selectively actuated to switch the resistors 106, 108, 110 into and out of the feedback circuit, based on the level of the current Iout. Typically, only one resistor 106, 108, 110 is switched into the feedback circuit at a time. If the current Iout increases, a smaller resistor can be switched into the feedback circuit and a larger resistor switched out, to keep the voltage VI within the desirable range. However, the amplifier 101 may saturate before the smaller resistor is switched into the feedback circuit, which is not desirable. If the current Iout decreases, a larger resistor can be switched into the feedback circuit and a smaller resistor out. Switching in a larger resistor requires a sudden change in the voltage VI at the output of the amplifier 101 and results in an undesirable glitch at an output node 112 (Vout) of the circuit.